Forced air is a furnace with fan running to force the air. I love my hot water rads they heat well and hold the temp. well and I would never go back to forced air with all the hot and cold spots . As for infloor heating that is lovely but to add it to an upper floor is costly and you will probably need a new boiler to handle the new addition . What you need is a really good HVAC person to advise on what you can have and what the price is .